In telecommunication systems, the end portions of the electrical path between subscriber premises are typically served by multipair cables. Each cable generally comprises numerous conductors arranged as twisted pairs which are encompassed by a protective sheath. The conductors of a pair--referred to as the tip and ring conductors--constitute a two-wire transmission line which is utilized for communication and signaling as well as testing purposes.
Various types of telephone equipment terminate the different transmission lines serving the customer premises. Of course, one standard termination is a telephone set. Although there are presently numerous styles and types of telephone sets, the fundamental principle of operation for each in effecting transmission is the same, namely, modulating a DC line current via a pressure transducer, typically a carbon microphone, to generate an electrical signal on the line.
During the course of usage of a telephone set by a customer, situations occur wherein a telephone receiver may be either intentionally or inadvertently removed from the switchhook for prolonged periods without a dialing activity. Such a condition is referred to as a ROH (receiver-off-hook) fault. In implementing a subscriber line testing strategy to detect or locate line faults, it is important to distinguish a ROH fault from a resistive fault that may occur along the cable route. Repair of cable faults is labor intensive, so considerable savings will accrue if a false dispatch of cable repair personnel can be eliminated. One such false dispatch is eliminated if a ROH condition is correctly diagnosed.
One conventional method of detecting a ROH condition utilizes an AC methodology. Briefly, with this method, a tone in the voice band is applied to the suspect line and the signal magnitudes at the second and third harmonic frequencies of the tone are measured. These magnitudes are then compared to preselected threshold values to determine a potential ROH condition. Because a telephone set with a receiver-off-hook exhibits a nonlinear signal response characteristic, harmonic levels above the threshold indicate a ROH. This is in contrast to resistive fault characteristics which are basically linear over the measurement interval.
One shortcoming of the AC method is the low reliability of the harmonic information whenever AC noise is present on the line. Another deficiency is the inability to distinguish a ROH condition from a tone ringer since a tone ringer returns harmonics similar to those of an off-hook telephone set. With the proliferation of tone ringers, this inability to differentiate presents a serious limitation on the future use of the AC method.